Characterization of a transient outward current in a rapidly adapting insect mechanosensory neuron

This paper describes the first voltage-clamp recordings from an arthropod cuticular sensory neuron. In the femoral tactile spine neuron of the cockroach Periplaneta americana, a rapidly activating and inactivating outward current, I_A, appeared when the neuron was hyperpolarized for a short period before a depolarizing test pulse. I_A could be separated from the other outward currents using 5 mM 4-aminopyridine (4-AP), which specifically blocked it. Tetraethylammonium (TEA), (50 mM) did not remove I_A, but decreased the steady-state outward current by about 50%. The threshold for I_A activation was about -75 mV. The minimum activation and inactivation time constants were approximately 0.2 ms and 15 ms, respectively. The voltage dependencies of activation and inactivation were well fit-ted by Boltzmann distributions, giving values of membrane potential at halfmaximal activation (V₅₀) equal to −56.5 mV and an equivalent gating charge of n=3.9 for activation and V₅₀=-86.7 mV and n=3.4 for inactivation. In current-clamp recordings, 4-AP reversibly reduced the cell's normal adaptation by lowering the threshold for action potentials, but did not affect the amplitude or duration of single action potentials. These results indicate that I_A plays a role in short-term adaptation by opposing membrane depolarization and reducing the spike frequency during maintained stimulation.